PHYSICAL AND CHEMICAL EXAMINATION OF MILK 197 



haps to dissolved carbon dioxid gas. The amount of bound acid 

 thus found is usually from 0.07 to 0.1 per cent., expressed in 

 lactic acid. To litmus paper fresh milk reacts amphoteric by 

 turning blue paper red and red paper slightly blue. 



Within a short time after milking the acidity increases per- 

 ceptibly, due to bacterial activity. The increase during the 

 first few hours is usually slow, but soon becomes more marked. 

 The degree of bacterial contamination and the temperature at 

 which the milk is kept are the chief factors influencing acid forma- 

 tion. Therefore the amount of acid depends in a measure on the 

 cleanliness of production and the temperature at which milk is 

 kept. For this reason the determination of acid in milk is often 

 an important factor in judging the quality of milk. 



Furthermore, tests for acidity are of value to the butter and 

 cheese makers, whose products are largely influenced by the 

 amount of acid present in cream and milk. According to the 

 richness of cream an acidity of 0.5 to 0.7 per cent, is most con- 

 ductive to yield butter of good quality. 



Acidity in milk can be measured by several methods which 

 depend upon neutralizing the acid with solutions of sodium hy- 

 drate, the neutral point being indicated by the use of phenol- 

 phthalein. Soxhlet mixed 2 c.c. of a 2 per cent, alcoholic solution 

 of phenolphthalein with 50 c.c. of milk and titrated this mixture 

 with one-fourth normal sodium hydrate solution. The number of 

 cubic centimeters of the one-fourth normal sodium hydrate solu- 

 tion necessary to cause the first permanent pink color to appear, 

 multiplied by 2, were recorded as degrees of acidity. In this 

 country the most popular tests are those of Manns, Van Norman, 

 Publow, Marshall, and Farrington. 



Manns' Test. — The "neutrahzer" used in Manns' test is a 0.1 

 normal sodium hydrate solution and the indicator is prepared by 

 dissolving 10 grams phenolphthalein in 300 c.c. 90 per cent, alcohol. 

 The milk is measured in a 50-c.c. pipet. The neutralizer is slowly 

 discharged from a buret into the milk containing a few drops of the 

 indicator. The result is expressed in per cent, lactic acid and is 

 obtained by multiplying the number of cubic centimeters of the 

 neutralizer used by 0.009, dividing the result by the number of 

 cubic centimeters of the sample, and then multiplying this result 

 by 100. When 50 c.c. milk are used the number of cubic centi- 

 meters of neutralizer used is multiplied by 0.018 (Fig. 69). 



Van Norman's Test. — The milk is measured in a 17.6-c.c. 

 pipet, placed in a cup, a few drops of phenolphthalein solution 

 added, and the mixture titrated with 0.02 normal sodium hydrate 

 solution. Each cubic centimeter of the sodium hydrate solution 

 corresponds to 0.01 per cent. acid. 



